Principled async operations for the store layer via C++20 stackless coroutines

[xokdvium]

Is your feature request related to a problem?

Currently nix store layer has a lot of ad-hoc callback-based API combined with rather unstructured usage of ThreadPool to prevent blocking on high-latency operations (like queryPathInfo). Callback-based API is hard to write and maintain.

There are a lot of use-cases that would benefit from having a single principled approach to asynchronous operations.

Some use-cases:

• Add paths to the store asynchronously #13798
• libutil: Use ThreadPool in computeClosure to parallelize promise resolution #14604
• Short-circuit computeFSClosure when copying #14569

• nix/src/libstore/build/substitution-goal.cc

  Lines 94 to 95 in 152e7e4

  	// FIXME: make async
  	info = sub->queryPathInfo(subPath ? *subPath : storePath);

• nix/src/libstore/remote-fs-accessor.cc

  Lines 30 to 31 in 152e7e4

  	/* FIXME: do this asynchronously. */
  	writeFile(makeCacheFile(hashPart, "nar"), nar);

• 95bd5da
• Make number of parallel substituter queries configurable #5324

Proposed solution

Leverage C++20 stackless coroutines to replace ad-hoc async. This is what the Lix project has done with the kj-async framework. Our goal scheduling already uses raw C++20 coroutine primitives for build scheduling.

Since C++ standard library currently lacks well-supported facilities for "Task", "Generator" types (see std::generator in https://en.cppreference.com/w/cpp/compiler_support.html). "Task" type is set to be added in C++26 https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2025/p3552r0.pdf.
Thus we need to rely on a library that provides those types or wait forever until C++ stdlib gets enough support. A non-exhaustive list of them is (in the order of being well-supported):

• kj-async (this is what lix project landed on)
• Boost.Cobalt recently merged and available since 1.87. Has avenues to integrate with ASIO.
• libcoro
• cppcoro - abandonware, just adding this for completeness sake.

Alternative solutions

Continue writing ad-hoc solutions and fix performance issues with threading.

Additional context

Checklist

• checked latest Nix manual (source)
• checked open feature issues and pull requests for possible duplicates

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